CHAPTER 14
CABIN ATMOSPHERE CONTROL SYSTEM
NEED FOR OXYGEN Oxygen is essential for most living processes.
Without oxygen, men and other animals die very rapidly. But before this extreme state is reached, a reduction in normal oxygen supplies to the tissues of the body can produce important changes in body functions, thought processes, and degree of consciousness. The sluggish condition of mind and body caused by a deficiency or lack of oxygen is called hypoxia. There are several causes of hypoxia, but the one which concerns aircraft operations is the decrease in partial pressure of the oxygen in the lungs.
The rate at which the lungs absorb oxygen depends upon the oxygen pressure. The pressure that oxygen exerts is about one-fifth of the total air pressure at any one given level. At sea level, this pressure value (3 p.s.i.) is sufficient to saturate the blood. However, if the oxygen pressure is reduced, either from the reduced atmospheric pressure at altitude or because the percentage of oxygen in the air breathed decreases, then the quantity of oxygen in the blood leaving the lungs drops and hypoxia follows.
From sea level to 7,000 ft. above sea level, the oxygen content and pressure in the atmosphere remain sufficiently high to maintain almost full saturation of the blood with oxygen and thus ensure normal body and mental functions.
At high altitude there is decreased barometric pressure, resulting in decreased oxygen content of the inhaled air. Consequently, the oxygen content of the blood is reduced.
At 10,000 ft. above sea level oxygen saturation of the blood is about 90%. Long exposure at this altitude will result in headache and fatigue. Oxygen saturation drops to 81% at 15,000 ft. above sea level. This decrease results in sleepiness, headache, blue lips and fingernails, impaired vision and judgment, increased pulse and respiration, and certain personality changes.
At 22,000 ft. above sea level the blood saturation is 68% and convulsions are likely to occur. Remaining without an oxygen supply at 25,000 ft. for 5 minutes where the blood saturation is down to 55 to 50% will cause unconsciousness.
COMPOSITION OF THE ATMOSPHERE The mixture of gases commonly called air but more technically termed atmosphere is composed principally of nitrogen and oxygen, but there are smaller quantities of other important gases, notable carbon dioxide, water vapor, and ozone. Figure 14-1 indicates the respective percentage of the quantity of each gas in its relation to the total mixture.
Other gases 1% Oxygen 21% Nitrogen 78% FICURE 14-1. The gases of the atmosphere.
As the altitude increases, the total quantity of all the atmospheric gases reduces rapidly, and, except for water vapor and ozone, the relative proportions of the gaseous mixture remain unchanged up to about 50 miles altitude, or slightly above. Above 50 miles altitude, changes do take place, and different 539
gases and new forms of the gases present at lower.
altitudes appear.
Nitrogen is the most common gas and comprises 78% of the total mixture of atmospheric gases.
However, insofar as man is concerned nitrogen is an inert gas which cannot be used directly for his own life processes. But biologically it is of immense importance because many compounds containing nitrogen are essential to all living matter.
Oxygen and its importance cannot be overestimated. Without oxygen, life as we know it cannot exist. Oxygen occupies 21% of the total mixture of atmospheric gases.
Carbon dioxide is of biological interest. The small quantity in the atmosphere is utilized by the plant world to manufacture the complex substance which animals use as food. Carbon dioxide also helps in the control of breathing in man and other animals.
Water vapor in the atmosphere is variable, but, even under the moist conditions at sea level, it rarely exceeds 5%; yet this gas absorbs far more energy from the sun than do the other gases. Vapor is not the only form in which water occurs in the atmosphere; water and ice particles are nearly always present. These ice particles also absorb energy and, with water vapor, play an important part in the formation of atmospheric and weather conditions.
Ozone is a variety of oxygen which contains three atoms of oxygen per molecule rather than the usual two. The major portion of the ozone in the atmosphere is formed by the interaction of oxygen and the sun's rays near the top of the ozone layer.
Ozone is also produced by electrical discharges, and the peculiar odor of ozone, which is somewhat like that of weak chlorine, can be detected after lightning storms. Auroras and cosmic rays may also produce ozone. Ozone is of great consequence to both living creatures on earth and to the circulation of the upper atmosphere. Ozone is important to living organisms because it filters out most of the sun's ultraviolet radiation.
Pressure of the Atmosphere The gases of the atmosphere (air), although invisible, have weight just like that of solid matter.
The weight of a column of air stretching from the surface of the earth out into space is called the atmospheric pressure. If this column is 1 sq. in., the weight of air at sea level is approximately 14.7 lbs.;
and the atmospheric pressure, therefore, can be stated as 14.7 p.s.i. at sea level. Another common way of stating the atmospheric pressure is to give the height of a column of mercury which weighs the same as a column of the atmosphere of the same cross sectional area. When measured this way, the atmospheric pressure at sea level is normally 1013.2 millibars, or 29.92 in. Hg.
The atmospheric pressure decreases with increasing altitude. The reason for this is quite simple: the column of air that is weighed is shorter. How the pressure changes for a given altitude is shown in figure 14-2. The decrease in pressure is a rapid one, and at 50,000 feet the atmospheric pressure has dropped to almost one-tenth of the sea level value, At a few hundred miles above the earth, the air has become so rarefied (thin) that the atmosphere can be considered nonexistent, but the line of demarcation with space is very vague.
100,000 80,000 60,000 40,000 20,000 Sea levei 10 12 Pressure in pounds per square inch FIcURE 14-2. How the atmospheric pressure decreases with altitude. For example, at sea level the pressure is 14.7
p.s.i; while at 40,000 ft., as the dotted lines show, the pressure is only 2.72 p.a.i.
Temperature and Altitude The variations in atmospheric temperature near the earth are well known and need no discussion.
However, at high altitudes the atmospheric temperature is not so variable but tends to have a more set pattern.
The meteorologist finds it convenient to define, somewhat arbitrarily, the atmosphere as being made up of several layers. The lowest of these is called the troposphere. The air temperature decreases with 540
increasing altitude in the troposphere and reaches a definite minimum at the top of the layer. The top of the troposphere is called the tropopause. The tropopause reaches its greatest height over the equator (about 60,000 ft.) and its lowest height over the poles (about 30,000 ft.). The tropopause marks the point at which air temperature stops decreasing with increasing altitude, and remains essentially constant.
The atmospheric layer above the tropopause is called the stratosphere. The lower stratosphere is an isothermal (constant temperature) region in which the temperature does not vary with altitude. The isothermal region continues up to about 82,000 to 115,000 ft. altitude. Above this level, the temperature increases sharply at the rate of about 1.5° C per 1,000 ft. The temperature reaches a peak at about 164,000 to 197,000 ft. altitude. Above the 197,000 ft. altitude (approximately), the temperature decreases again, reaching a minimum of -10° F. to -100° F. at about 230,000 to 262,000 ft. altitude. Above this level, the temperature again increases and apparently continues to increase until the edge of space.
The foregoing paragraphs have presented a general knowledge of the atmosphere. It is obvious that a means of preventing hypoxia and its ill effects must be provided. When the atmospheric pressure falls below 3 p.s.i. (approximately 40,000 ft.), even breathing pure oxygen is not sufficient.
The low partial pressure of oxygen, low ambient air pressure, and temperature at high altitude make it necessary to create the proper environment for passenger and crew comfort. The most difficult problem is maintaining the correct partial pressure patiene of oxygen in the inhaled air. This can be achieved by using oxygen, pressurized cabins, or pressure suits. The first and second methods are used extensively in civil aviation.
Pressurization of the aircraft cabin is now the accepted method of protecting persons against the effects of hypoxia. Within a pressurized cabin, people can be transported comfortably and safely for long periods of time, particularly if the cabin altitude is maintained at 8,000 ft., or below, where the use of oxygen equipment is not required. However, the flight crew in this type of aircraft must be aware of the danger of accidental loss of cabin pressure and must be prepared to meet such an emergency whenever it occurs.
PRESSURIZATION When an aircraft is flown at a high altitude, it burns less fuel for a given airspeed than it does for the same speed at a lower altitude. In other words, the airplane is more efficient at a high altitude. In addition, bad weather and turbulence can be avoided by flying in the relatively smooth air above the storms. Aircraft which do not have pressurization and air conditioning systems are usually limited to the lower altitudes.
A cabin pressurization system must accomplish several functions if it is to assure adequate passenger comfort and safety. It must be capable of maintaining a cabin pressure altitude of approximately 8,000 ft. at the maximum designed cruising altitude of the aircraft. The system must also be designed to prevent rapid changes of cabin altitude which may be uncomfortable or injurious to passengers and crew. In addition, the pressurization system should permit a reasonably fast exchange of air from inside to outside the cabin. This is necessary to eliminate odors and to remove stale air.
In the typical pressurization system, the cabin, flight compartment, and baggage compartments are incorporated into a sealed unit which is capable of containing air under a pressure higher than outside atmospheric pressure. Pressurized air is pumped into this sealed fuselage by cabin superchargers which deliver a relatively constant volume of air at all altitudes up to a designed maximum. Air is released from the fuselage by a device called an outflow valve. Since the superchargers provide a constant inflow of air to the pressurized area, the outflow valve, by regulating the air exit, is the major controlling element in the pressurization system.
The flow of air through an outflow valve is determined by the degree of valve opening. This valve is ordinarily controlled by an automatic system which can be set by the flight crewmembers. A few simple minor adjustments are required on the average flight, but most of the time automatic controls need only to be monitored. In the event of a malfunction of the automatic controls, manual controls are also provided. A schematic of a basic pressurization system is shown in figure 14-3.
The degree of pressurization and, therefore, the operating altitude of the aircraft are limited by several critical design factors. Primarily the fuselage is designed to withstand a particular maximum cabin differential pressure. Cabin differential pres- 541
- Pressure relief valves Negative pressure valve -
- Cable control Air-conditioning unit Cabin pressure control valve.
Supercharger' Supercharger Cabin air Cabin air inlet scoop -Combustion air inlet scoop Emergency depressurization valve inlet for cabin heater Emergency cabin altitude control- • Pressurized area Emergency depressurization control FICURE 14-3. Basic pressurization system.
sure is the ratio between inside and outside air pressures and is a measure of the internal stress on the fuselage skin. If the differential pressure becomes too great, structural damage to the fuselage may occur. In addition, pressurization is limited by the capacity of the superchargers to maintain a 542
constant volume of airflow to the fuselage. As altitude is increased, the pressure of the air entering the supercharger becomes less; consequently, the superchargers have to work harder to accomplish their part of the job. Eventually at some high altitude the superchargers will reach their designed limit of speed, power absorbed, or some other operating factor. The aircraft will normally not be flown higher than these limits allow.
Pressurization Problems There are many complex technical problems associated with pressurized aircraft. Perhaps the most difficult problems are in the design, manufacturing, and selection of structural materials which will withstand the great differential in pressure that exists between the inside and outside of a pressurized aircraft when flying at high altitudes. If the weight of the aircraft structure were of no concern, it would be a relatively simple matter to construct a fuselage which could withstand tremendous pressures.
It is necessary to construct a fuselage capable of containing air under pressure, yet be light enough to allow profitable loading. As a general rule pressurized aircraft are built to provide a cabin pressure altitude of not more than 8,000 ft. at maximum operating altitude. If an aircraft is designed for operation at altitudes over 25,000 ft., it must be capable of maintaining a cabin pressure altitude of 15,000 ft., in the event of any reasonably likely failure.
The atmospheric pressure at 8,000 ft. is approximately 10.92 p.s.i., and at 40,000 ft. it is nearly
2.72 p.s.i. If a cabin altitude of 8,000 ft. is maintained in an aircraft flying at 40,000 ft. the differential pressure which the structure will have to withstand is 8.20 p.s.i. (10.92 p.s.i. minus 2.72
p.s.i.). If the pressurized area of this aircraft contains 10,000 sq. in., the structure will be subjected to a bursting force of 82,000 lbs., or approximately 41 tons. In addition to designing the fuselage to withstand this force, a safety factor of 1.33 must be added. The pressurized portion of the fuselage will have to be constructed to have an ultimate strength of 109,060 lbs. (82,000 times 1.33), or 54.5 tons.
From the foregoing example it is not difficult to grasp an idea of the difficulties encountered in designing and building a fuselage structure which will be light enough and strong enough at the same time.
AIR CONDITIONING AND PRESSURIZATION SYS- TEMS The cabin air conditioning and pressurization system supplies conditioned air for heating and cooling the cockpit and cabin spaces. This air also provides pressurization to maintain a safe, comfortable cabin environment. In addition to cabin air conditioning, some aircraft equipment and equipment compartments require air conditioning to prevent heat buildup and consequent damage to the Some of the air conditioning systems installed in modern aircraft utilize air turbine refrigerating units to supply cooled air. These are called air cycle systems. Other model aircraft utilize a compressed gas cooling system. The refrigerating unit is a freon type, quite similar in operation to a common household refrigerator. Systems utilizing this refrigera.
tion principle are called vapor cycle systems.
Terms and Definitions The system which maintains cabin air temperatures is the air conditioning system. The sources of heat which make cabin air conditioning necessary are: (1) Ram-air temperature, (2) engine heat, (3) solar heat, (4) electrical heat, and (4) body heat.
It is necessary to become familiar with some terms and definitions to understand the operating principles of pressurization and air conditioning systems. These are:
(1) Absolute pressure. Pressure measured along a scale which has zero value at a complete vacuum.
(2) Absolute temperature. Temperature measured along a scale which has zero value at that point where there is no molecular motion (-273.1° C. or -459.6° F.).
(3) Adiabatic. A word meaning no transfer of heat. The adiabatic process is one in which no heat is transferred between the working substance and any outside
(4) Aircraft altitude. The actual height above sea level at which an aircraft is flying.
(5) Ambient temperature. The temperature in the area immediately surrounding the object under discussion.
(6) Ambient pressure. The pressure in the area immediately surrounding the object under discussion.
(7) Standard barometric pressure. The weight of gases in the atmosphere sufficient to 543
Flight deck Foot warmers supply Floor outlets Roof outlet External conditioned Conditioned air to cabin Regulating valve air supply through wall outets Temperature Ram Temperature control control valve To cold air louvers air valve valve Ram air V Ram air Water Water separators: separators Refrigeration Refrigeration unit unit Heat exchangers Refrigeration bypass Refrigeration valve bypass valve Flow control valve Automatic shutters Flow control valve To wing anti-icing To tail anti-icing Heat exchanger Heat exchanger Crossfeed valve / Ground connection Pressure Pressure relief Pressure reducing reducing Pressure relief valve valve valve valve To water injection To water injection turbopump turbopump Isolation valve Isolation valve Air delivery No. 1 valve No. 2 Engine Engine A.P.U.
Constant speed Constant speer drive and starter drive and starter Hot air Conditioned air Cold air Cooling air FIGURE 14-4. Typical pressurization and air conditioning system.
hold up a column of mercury 760 milimeters high (approximately 30 in.) at sea level (14.7 p.s.i.). This pressure decreases with altitude.
(8) Cabin altitude. Used to express cabin pressure in terms of equivalent altitude above sea level.
(9) Differential pressure. The difference in pressure between the pressure acting on one side of a wall and the pressure acting on the other side of the wall. In aircraft air conditioning and pressurizing systems, it is the difference between cabin pressure and atmospheric pressure.
(10) Gage pressure. A measure of the pressure in a vessel, container, or line, as compared to ambient pressure.
(11) Ram-air temperature rise. The increase in temperature created by the ram compression on the surface of an aircraft traveling at a high rate of speed through the atmosphere. The rate of increase is proportional to the square of the speed of the object.
(12) Temperature scales.
(a) Centrigrade. A scale on which 0° C.
represents the freezing point of water, and 100° C. is equivalent to the boiling point of water at sea level.
(b) Fahrenheit. A scale on which 32° F.
represents the freezing point of water, and 212° F. is equivalent to the boiling point of water at sea level.
BASIC REQUIREMENTS Five basic requirements for the successful functioning of a cabin pressurization and air conditioning system are:
(1) A source of compressed air for pressurization and ventilation. Cabin pressurization sources can be either engine-driven compressors, independent cabin superchargers, or air bled directly from the engine,
(2) A means of controlling cabin pressure by regulating the outflow of air from the cabin. This is accomplished by a cabin pressure regulator and an outflow valve.
(3) A method of limiting the maximum pressure differential to which the cabin pressurized area will be subjected. Pressure relief valves, negative (vacuum) relief valves, and dump valves are used to accomplish this.
(4) A means of regulating (in most cases cooling) the temperature of the air being distributed to the pressurized section of the airplane. This is accomplished by the refrigeration system, heat exchangers, control valves, electrical heating elements, and a cabin temperature control system.
(5) The sections of the aircraft which are to be pressurized must be sealed to reduce inadvertent leakage of air to a minimum.
This area must also be capable of safely withstanding the maximum pressure dif ferential between cabin and atmosphere to which it will be subjected.
Designing the cabin to withstand the pressure differential and hold leakage of air within the limits of the pressurization system is primarily an airframe engineering and manufacturing problem.
In addition to the components just discussed, various valves, controls, and allied units are necessary to complete a cabin pressurizing and air conditioning system. When auxiliary systems such as windshield rain-clearing devices, pressurized fuel tanks, and pressurized hydraulie tanks are required, additional shutoff valves and control units are neces.
sary.
Figure 14-4 shows a schematic diagram of a pressurization and air conditioning system. The exact details of this system are peculiar to only one model of aircraft, but the general concept is similar to that found in the majority of aircraft.
SOURCES OF CABIN PRESSURE Reciprocating engine internal superchargers provide the simplest means of cabin pressurization.
This is accomplished by ducting air from a manifold which supplies compressed air from a supercharger to the pistons. This arrangement can be used only when the engine carburetor is downstream of the supercharger. When the carburetor is upstream of the supercharger, as is often the case, this method cannot be used since the compressed air contains fuel. Air for cabin pressurization can also be ducted from a turbocharger used with a reciprocating engine.
There are several disadvantages in using these two methods. The cabin air becomes contaminated with fumes from lubricating oil, exhaust gases, and fuel. Also, cabin pressurization at high altitude be- 545
comes impossible as the discharge pressure of the supercharger decreases to nearly ambient. A third disadvantage is the decrease in engine performance near its design ceiling due to the air loss for cabin pressurization.
With gas turbine engines the cabin can be pressurized by bleeding air from the engine compressor.
Usually the air bled from an engine compressor is sufficiently free from contamination and can be used safely for cabin pressurization. Even so, there are several disadvantages when using bleed air from turbine engine compressors. Among these disadvantages are: (1) The possibility of contamination of the air from lubricants or fuel in the event of leakage, and (2) dependence of the air supply on the engine performance.
Because of the many disadvantages associated with the pressurizing sources previously described, independent cabin compressors have been designed.
These compressors can be engine driven through accessory drive gearing or can be powered by bleed air from a turbine engine compressor.
Generally, the compressors can be separated into two groups, (1) positive-displacement compressors and (2) centrifugal compressors.
Positive-Displacement Cabin Compressors (Superchargers) Included in this group are reciprocating compres sors, vane-type compressors, and Roots blowers.
The first two are not very suitable for aircraft cabin pressurization because of the large quantity of oil present in the air delivered to the cabin.
The action of a Roots-type blower (figure 14-5) is based on the intake of a predetermined volume of air, which is subsequently compressed and delivered to the cabin duct.
The rotors are mounted in an airtight casing on two parallel shafts. The lobes do not touch each other or the casing, and both rotors turn at the same speed. Air enters the spaces between the lobes, is compressed, and is delivered to the cabin air duct.
A cutaway view of a cabin supercharger is shown in figure 14-6. The supercharger housing is usually finned on the external surface to increase its cooling area. The cooling effect is sometimes further increased by shrouding the supercharger housing and passing a stream of air through it. Air cooling is also used to reduce the temperature of internal parts. The cooling air is ducted through drilled FIGURE 14-5. Schematic Roots-type cabin compressor.
passageways into the rotor cavities and is expelled at the inlet side of the supercharger cover.
To achieve an oil-free delivery of air, the supercharger bearings are contained in separate chambers. The rotor shafts can be fitted with seals made of oil-resistant rubber which prevents any lubricant from entering the compressor casing. The use of labyrinth seals permits a small amount of air leakage to ambient. Any drops of oil which may have passed the rubber seal are thus blown back.
Positive displacement compressors emit a shrill noise during their operation, because of the air pulsations caused by the rotors. Silencers are used with this type compressor to reduce the noise level.
Centrifugal Cabin Compressors The operating principle of a centrifugal compressor is based on increasing the kinetic energy of the air passing through the impeller. With compressor impeller rotation, the induced air is not only accelerated, but it is also compressed because of the action of centrifugal force. The kinetic energy in the air is then converted into pressure in the diffuser. There are two basic types of diffusers: (1) Vaneless, where the air enters the diffuser space directly on leaving the impeller, and (2) those having guide vanes. A schematic of a centrifugal cabin compressor is shown in figure 14-7.
The cabin supercharger shown in figure 14-8 is essentially an air pump. It incorporates a centrifugal impeller similar to the supercharger in the induction system of a reciprocating engine. Outside 546
Air inlet port Driving rotor Rotor case Drive end cover Driving gear Extractor seal shell Rotary shell Extractor scroll Driven rotor Stub shaft Adjusting ring Abutment collar Driven gear Ball bearing FIcURE 14-6. Cutaway view of a Roots-type cabin supercharger.
air at atmospheric pressure is admitted to the supercharger by suitable scoops and ducts. This air is then compressed by the high-speed impeller and delivered to the fuselage. The superchargers are usually driven by the engine through appropriate gearing; however, turbojet aircraft utilize superchargers (turbocompressors) which are pneumatically driven.
Engine-driven cabin superchargers are generally mounted in the engine nacelle. The supercharger is either splined directly to the engine accessory drive or is connected to an accessory drive by a suitable drive shaft. A mechanical disengaging mechanism is usually incorporated in the drive system to permit disconnecting the supercharger if it malfunctions.
The disengaging mechanism can be operated from the flight deck by the crew. In most aircraft it is not possible or permissible to re-engage the supercharger in flight once it has been disconnected.
FIGURE 14-7. Centrifugal cabin compressor. Engine-driven superchargers used on reciprocat- 547
FIGURE 14-8. Pictorial view of a centrifugal cabin supercharger.
ing engine aircraft require a variable-ratio drive mechanism. The gear ratio of these superchargers is automatically adjusted to compensate for changes of engine r.p.m. or outside atmospheric pressure. Normally, the gear ratio is eight to 10 times engine speed when operating under cruising conditions.
The drive ratio is at a maximum when operating at high altitude with low engine r.p.m.
Turbocompressors used on turbojet aircraft may be located in the engine nacelles or in the fuselage.
There may be as many as four turbocompressors in an aircraft. Turbocompressors consist of a turbine rotated by air pressure which in turn rotates an impeller. The compressed air supply used to operate the turbocompressor is taken from the aircraft's pneumatic system. Speed of the turbocompressor is controlled by varying the supply of compressed air to its turbine.
Cabin superchargers of all types contain their own lubrication system. The lubricant used may be the same type oil used for the engine, or it may be a special oil similar to hydraulic fluid. Supercharger bearings and gears are lubricated by pressure and spray. Oil pressure is also used to operate the control system for the supercharger. The typical lubrication system incorporates a pump, relief valve, sump, cooling system, and sometimes a separate oil tank.
High impeller speed is an important limitation in all superchargers. When the tip speed of the impeller approaches the speed of sound, the impeller rapidly loses its efficiency as an air pump. An equally important limitation involves the back pressure created in the outlet air ducts. If the back pressure is excessive, the impeller may stall or surge.
Supercharger Control The function of the supercharger control system is to maintain a fairly constant volume of air output from the supercharger. This is accomplished in the system used on reciprocating engine aircraft by varying the drive ratio of the supercharger. The drive ratio between the supercharger impeller and the engine is varied to compensate for changes in engine r.p.m. or atmospheric pressure. This is achieved by an automatic mechanism which samples the airflow output of the supercharger and, through a variable-speed drive gearbox, adjusts the impeller speed whenever the airflow output varies from a preset value.
The amount of f.hp. (friction horsepower) taken from the engine to drive the supercharger is dependent upon the drive ratio. Losses are lowest during low-ratio operation when the energy required to rotate the impeller is at a minimum.
Losses are approximately 75 f.hp. in high ratio and 25 f.hp. in low ratio. This loss is indicated at high altitudes, where, the engines which drive the cabin superchargers may require 3 or 4 in. Hg additional manifold pressure to produce the same b.hp. (brake horsepower) as that of other engines.
The speed of the supercharger impeller is therefore adjusted by the control system to maintain a constant mass airflow output. If variables such as altitude tend to increase or decrease the output, the control mechanism causes a correction of the drive ratio. Changes of drive ratio are furthermore dampened by various system refinements to prevent rapid acceleration or deceleration which may result in uncomfortable surges of pressurization.
Serious consequences may occur if the impeller speed becomes higher than its designed maximum.
To protect the supercharger against such an occurrence, the typical system has an overspeed governor.
This unit is similar to a propeller flyweight governor. The overspeed governor actuates a valve to position the control mechanism to the low-ratio po- 548
sition. It works automatically to reduce impeller
r.p.m. when an overspeed occurs.
Some installations also have an electrically operated valve which positions the control mechanism to the low-speed position. This minimum speed valve may be operated manually from the flight deck or automatically by a landing gear strut switch. It is used primarily to reduce supercharger drive ratio when pressurization is not being used or when emergencies occur.
SUPERCHARGER INSTRUMENTS The principal instrument associated with the supercharger is an airflow gage. This gage usually measures the differential air pressure between the input and the output of the supercharger. In some cases there are two needles on the gage to indicate input and output pressures on the same scale. The airflow (or input and output pressure) gage indicates the proper operation of the supercharger.
High readings, low readings, or fluctuating readings indicate various types of malfunctions.
Oil pressure and oil temperature indications are also made available by suitable instruments on the fight deck. In some cases warning lights may be used instead of, or in addition to, the actual gages.
Engine-driven cabin compressors are used on turboprop aircraft. These compressors do not have a variable-speed drive because the turboprop engine operates at a relatively constant speed. The output of this type compressor is controlled by automatically varying the inlet airflow through an airflowsensing mechanism and a suitable inlet valve which maintains a constant compressor airflow output.
Ordinarily a surge and dump valve is used at the outlet of the compressor. In some systems this is the only type of control employed for the compressor:
The surge and dump valve prevents surging of the compressor by partially reducing output pressure when system demands are heavy. The valve can also completely dump output pressure when the enginedriven compressor output is not needed. This valve can be operated from the flight deck and is also operated by various automatic control systems.
When the surge and dump valve is opened, the engine-driven cabin compressor output is dumped overboard through suitable ducts.
Instruments used in conjunction with the enginedriven compressor are similar to those used with the variable-speed supercharger. An inlet and discharge pressure gage measures compressor pressures. Compressor high oil temperature and low oil pressure are usually indicated by warning lights.
Turbocompressors used on turbojet aircraft are similar in operation to the exhaust-driven turbochargers used with some reciprocating engines.
Power derived from the aircraft's pneumatic system is used to drive the turbine of the unit. Since the turbocompressors do not rely upon direct engine drive shafts, they can be placed either in the engine nacelles or in the fuselage. Ordinarily multiple turbocompressor units are used to provide the high airflow needed by the large turbojet aircraft. The output of the turbocompressor units is usually controlled by varying the pneumatic supply to the turbine.
The pneumatic air supply is obtained from the compressor section of the turbojet engine. This air supply is regulated to a constant pressure of approximately 45 p.s.i. to 75 p.s.i. Pneumatic system air pressure is also used to operate anti-icing and other aircraft systems; therefore, various shutoff valves and check valves are used to isolate inoperative units of the turbocompressor system.
The turbocompressor output is controlled automatically by an airflow control valve and servo-operated inlet vanes. The inlet vanes control the pneumatic system air supply to the turbocompressor turbine. The vanes open or close according to the air pressure signal sensed at the airflow control valve, and turbocompressor speed is increased or decreased to maintain a relatively constant output air volume. Turbocompressor speed will therefore increase with altitude.
The principal turbocompressor control is a simple "on/off" valve. This valve is located in the pneumatic air duct. In the "off" position it completely closes off the pneumatic supply to the turbine. Various special circuits may also actuate this shutoff valve when operation of the turbocompressor is not desired.
Most turbocompressor units incorporate an overspecd control. A typical overspeed control unit is a simple flyweight governor which causes the turbocompressor to completely shut down when a certain limiting r.p.m. is reached. Usually the pneumatic duct shutoff valve is closed by the overspeed control. The turbocompressor system also uses a surge and dump valve similar to that used for enginedriven compressors.
The flight deck instruments are the same as those used on engine-driven systems with the addition of 549
a tachometer which measures turbocompressor speed. Turbocompressor speed on a typical aircraft varies from approximately 20,000 r.p.m. at sea level to 50,000 r.p.m. at 40,000 ft. The overspeed control may be set at about 55,000 r.p.m.
PRESSURIZATION VALVES The principal control of the pressurization system is the outflow valve. This valve is placed in a pressurized portion of the fuselage, usually underneath the lower compartments. The purpose of the valve is to vent cabin air overboard through suitable openings in the wing fillet or the fuselage skin. Small aircraft use one outflow valve; large aircraft may use as many as three valves which work in unison to provide the required volume outflow.
One type of outflow valve is a simple butterfly which is opened or closed by an electric motor. The motor receives amplified electrical signals from the pressurization controller to vary the valve position required for pressurized flight.
Some aircraft use a pneumatic outflow valve (figure 14-9). This valve receives signals from the pressurization controller in the form of controlled air pressures. The air pressures which operate the valve are obtained from the high pressure inside the cabin, with assistance from the pneumatic system pressure in turbine-powered aircraft.
FIGURE 14-9. Typical pneumatic outflow valve.
In many aircraft, the outflow valve(s) will be held fully open on the ground by a landing gear operated switch. During flight, as altitude is gained, the valve(s) close (s) gradually to make a greater restriction to the outfow of cabin air. The cabin rate of climb or descent is determined by the rate of closing or opening of the outflow valve (s). During cruising flight the cabin altitude is directly related to the degree of outflow valve opening.
In addition to the controllable outflow valve (s), an automatic cabin pressure relief valve is used on all pressurized aircraft. This valve may actually be built into the outflow valve or may be an entirely separate unit. The pressure relief valve automatically opens when the cabin differential pressure reaches a preset value.
All pressurized aircraft require some form of a negative pressure relief valve. This valve may also be incorporated into the outflow valve or may be an individual unit. A common form of negative pressure relief valve is a simple hinged flap on the rear wall (pressure dome) of the cabin. This valve opens when outside air pressure is greater than cabin pressure. During pressurized flight the internal cabin pressure holds the flap closed. The negative pressure relief valve prevents accidentally obtaining a cabin altitude which is higher than the aircraft altitude.
The outflow of air from the cabin can also be accomplished through a manually operated valve.
This valve may be called a safety relief valve, a manual depressurization valve, or some other similar term. The manual valve is used to control pressurization when all other means of control fail. It is primarily intended to permit rapid depressurization during fires or emergency descent.
Pressurization Controls The pressurization controller (figure 14-10) is the source of control signals for the pressurization system. The controller provides adjustments to obtain the desired type of pressurized condition. Most operators specify standard operating procedures for the controller which have proven best for their particular type of operation.
The controller looks very much like an altimeter which has several added adjustment knobs. The dial is graduated in cabin altitude increments up to approximately 10,000 ft. Usually there is one pointer which can be adjusted to the desired cabin altitude by the cabir altitude set knob. In some cases there is another pointer or a rotating scale which also 550